It is well known in the art to use a laser beam to etch a pattern onto a surface wherein the etched or engraved surface is then suitable for transferring this engraved pattern onto a material (usually thermoplastic) such that the surface of the product produced accurately represents a desired appearance and texture such as simulating wood grain, geometrical grain or leather. The depth of the cut is controlled by varying the intensity of the laser beam as the beam traverses the surface that is to be etched. Use of this technique enables man-made materials in situations requiring durable, cost effective materials to be used yet have the appearance of natural (or geometrical) obtained counterparts. In fact, vinyl siding, for example, can be manufactured that features a “leather” appearance and feel, yet has the advantages of superior durability.
Prior art techniques initially scanned the surface that was intended to be copied, such as leather or wood grain, to determine the position location and depth information which is then stored in a computer. The stored data is then processed to operate a laser beam that is directed onto the surface that is to be engraved. Representative of this technology is the method disclosed in U.S. Pat. No. 5,886,317, issued to Hinrichs et al. on Mar. 23, 1999. First the surface that is to be replicated is scanned, optically or mechanically, to obtain location-dependent depth information, which is then converted into electrical control signals, which can be imparted to a laser. Then the laser head emitting a single beam is either moved across the surface that is to be engraved if a flat surface is to be used. If a roller is to be engraved, the roller surface is rotated beneath the laser head to permit embossing. In both cases, the head is indexed across the surface so that the depth contour information can be etched onto the entire surface of the work piece. The finished work piece is then used as an embossing tool to replicate this surface onto the desired product such as vinyl siding. Typically, the surface that is scanned is substantially smaller than the surface area of the work piece so that the scanned surface must be multiplied accordingly. In order to prevent boundary lines between various “information areas” from being visually identified, this disclosure requires overlapping scanning using depth contour information at abutting boundaries to achieve cross-fading of the pattern in the boundary line regions. Thus, it is stated that this mirroring of adjacent sections using overlapping laser control signals will result in the reduction of visible boundary lines.
Using this technique to obtain fine reproductions of surfaces normally requires making multiple passes across the work piece surface since the amount of material that can be removed on a single pass is limited to what can be removed by the beam. To do otherwise, would be akin to attempting to carve a statute out of marble block to obtain the finished art on a single pass. Obviously, larger amounts of material can initially be removed but as the desired surface is being reached, the amount of material that can be removed must be increasingly less on each subsequent pass.
While the advantages of using laser engraving techniques are substantial with prior art methods, surface depth contour information must first be obtained by scanning the surface, as disclosed in Hinrichs. Also, the requirement of multiple passes over the work piece substantially increases the production time to obtain a finished work piece that is finely engraved. There is also difficulty experienced during the process of engraving rollers due to the relative pitch movements that can occur between the laser engraving head and the rotating roller that is being engraved. This pitch or screw-like motion that occurs due to the laser head linear motion relative to the rotating roller during the engraving process may compromise the quality of the engraving by having detectable pitch lines showing as spiral seams on the work piece.
There is not found in the prior art a method and an apparatus that eliminates the problems identified above.